Patent GB-1286687-A (GB)
Title: Electro-dynamic transducers
Inventor: MANGER JOSEF WIHELM
IPC: H04R7/02
Filed: 1969-12-19
Abstract: 1286687 Moving coil transducers; loudspeaker circuits; diaphragms J W MANGER 19 Dec 1969 [19 Dec 1968] 61989/69 Heading H4J A moving coil transducer, e.g. a loud-speaker, has the datum position of its diaphragm determined solely by datum currents flowing in two coils in associated magnetic fields. The two coils are mechanically coupled to one another and to the diaphragm. Two pot magnets 1, 1a (Fig. 1) are coaxially mounted by spindles 2, 2a rigidly clamped at their outer ends (not shown). Moving coils 5, 6 are movable in air gaps L 1 , L 2  and are rigidly connected together by an elliptical or spherical hollow body 7 which acts as a diaphragm, or on which a diaphragm is mounted. The hollow body 7 is slidable on the spindles 2, 2a, rubber stockings being provided which roll on the spindles 2, 2a as the body 7 moves to minimize friction. The datum position is determined by direct currents flowing through the coils, and audio frequency currents superimposed on these cause vibration of the diaphragm to provide loud-speaker operation. In a first alternative arrangement (Fig. 2, not shown), two pot magnets (8, 9) face each other. Thus two annular air gaps (L 1 , L 2 ) thus also face each other and each receives a moving coil (12, 13). The two moving coils (12, 13) are rigidly connected together by a hollow cylinder, or by two rods (16, 17), on which a diaphragm (11) is mounted. In another arrangement a single pot magnet (23), Fig. 4 (not shown), has two coils mounted in tandem in its air gap and these coils tend to be deflected in opposite direction by the datum direct current. In yet another arrangement (Fig. 6, not shown) the centring coils to which datum direct current is supplied are, separate from, but rigidly connected to, a voice coil which receives the audio frequency currents. Thus the two centring coils are mounted on a single former in air gaps provided by confronting pot magnets. The voice coil is situated in a homogeneous magnetic field provided by a third magnet. In all arrangements, the coils to which the datum direct currents are applied are either of non-uniform density of winding or the magnetic flux provided for these coils is inhomogeneously distributed, to ensure return of the diaphragm to its datum frequency in the absence of audio frequency currents. Local circuits.-The coil ends E 1 , E 2  (see Fig. 1) are connected together and to a battery negative terminal (Fig. 3). The coil ends A 1 , A 2  are connected to the collectors of a transistor pair 20, 21, whose emitters are connected to the battery positive terminal. The base electrodes of transistors 20, 21 are joined by the secondary of a transformer 22, the primary of which is provided with audio frequency currents. The datum position of the moving coils is determined by adjustable base bias resistors R 1 , R 2 . In another circuit (Fig. 5) an NPN transistor 24 and a PNP transistor 25 provide in-phase control. Two batteries B 1 , B 2  are provided, one for each transistor. Audio frequency signals are applied between the bases of the transistors 24, 25 and their emitters, which are connected to coil ends E 1 , E 2 .